Power line compensation is required to ensure that a sufficient amount of power is efficiently delivered from power generators to loads without causing overloads or other operating problems. Proper power system control reduces or eliminates such system problems as excessive line loadings, voltage transients and fluctuations and rapid changes in reactive power on the power system. These events may be caused by the effects of line switching, line faults, and rapidly varying active or reactive system loads.
During the past few decades, the increase in demand for electrical energy has imposed more stringent requirements on the power industry, requiring more power plants, substations and transmission lines. However, the most commonly used devices for controlling power systems have been mechanically-controlled circuit breakers. The “on/off” characteristics of these devices make them unsuitable for smoothly handling frequently-changing loads and damping transient oscillations. To overcome these drawbacks, substantial operating margins and redundancies are imposed to protect the power system from dynamic instabilities and to permit quick recovery after faults. These conditions increase the power system cost and complexity and lower system efficiency. Power system compensation can be divided into two categories, i.e., shunt compensation and series compensation.
A power system 10 comprises a generator 12 generating a voltage V at a phase angle of δ/2 and a generator 14 generating a voltage V at a phase angle of −δ/2. A transmission line reactance is segregated into two reactances Xl/2. The power system 10 further comprises shunt capacitance compensation in the form of a capacitor 20 connected at between a midpoint 21 (between the two generators 22 and 24) and ground. The voltage magnitude at the midpoint 21 is maintained at a voltage V. The capacitor 20 can inject an amount of reactive power given byQc=V2/Xc where V is the midpoint voltage, andXc is the capacitive reactance of the capacitor.
The capacitor helps maintain the voltage level on the transmission line by supplying reactive power to the transmission line. The ability to add reactive power increases the operational margin and the system stability.
Shunt reactive compensation can be similarly employed to consume (absorb) reactive power from the transmission line. Shunt-connected reactors reduce line over-voltage conditions by consuming reactive power. The reactors are controllable to control the amount of reactive power that is absorbed.
Shunt compensation, especially shunt reactive compensation, is widely used in transmission system to regulate the voltage magnitude, improve the voltage quality, and enhance system stability.
Series compensation attempts to directly control the overall series line impedance (Xl) of the transmission line. It can be shown that AC power transmission is limited primarily by the series reactive impedance of the transmission line. A series-connected capacitor can add a voltage in opposition to the transmission line voltage and thereby reduce the effective line impedance. A simplified model of a transmission system 30 with series compensation is shown in FIG. 2. The voltage magnitudes of the two buses are assumed equal (V) and the phase angle between them is designated δ. The transmission line is assumed lossless and represented by the reactance Xl. A controllable capacitor 32 is series-connected to the transmission line 34 and imposes a voltage Vc in opposition to the nominal transmission line voltage Vl.
The phasor diagram of FIG. 3 illustrates the associated phasors.
Lowering the line inductive reactance using series capacitors is an effective technique for increasing power transfer capability, improving transient and steady state stability, reducing rapid voltage fluctuations, and reducing line losses. These benefits are achieved because, as mentioned above, the series-connected capacitors partially compensate the inductive reactance of the transmission lines.